REVIEW on CARBOHYDRATES

1
REVIEW on CARBOHYDRATES

• Aulanniam
• Biochemistry Laboratory_UB

2
CARBOHYDRATES

• Hydrates of carbon Cn(H2O)m
• Polyhydroxyaldehyde or polyhydroxyketone, or
  substance that gives these compounds on
  hydrolysis
• Most abundant organic compound in the plant world
• Chemically made up of skeletal C,H which is
  usually 2x the number of C, highly variable
  number of O, occasional N S
• Linked to many lipids and proteins

3
FUNCTIONS of CARBOHYDRATES

• Storehouses of chemical energy
• Glucose,starch, glycogen
• Structural components for support
• Cellulose, chitin, GAGs
• Essential components of nucleic acids
• D-ribose, 2-deoxy-D-ribose
• Antigenic determinants
• Fucose, D-galactose, D-glucose,
  N-acetyl-D-glucosamine, D-acetyl-D-galactosamine

4
SPECIFIC CARBOHYDRATES

• Monosaccharides
• Glucose (dextrose, grape sugar, blood sugar)
• Can be stored as glycogen
• Most metabolically important monosaccharide
• Fructose (levulose)
• Galactose (brain sugar)
• Mannose
• Targets lysosomal enzymes to their destinations
• Directs certain proteins from Golgi body to
  lysosomes

5

• Disaccharides
• Sucrose (table sugar, cane sugar, saccharose)
• glucose fructose linked aß1-2
• Lactose (milk sugar) glu gal linked ß 1-4
• Maltose (malt sugar) 2 glucose linked a 1-4
• Trehalose (mycose) 2 glucose linked a 1-1
• Gentiobiose (amygdalose) 2 glucose linked ß 1-6
• Cellobiose 2 glucose linked ß 1-4

6
CLASSES OF CARBOHYDRATES

• Number of C
• Triose, tetroses, pentose, hexose, heptulose
• Number of saccharide units
• Monosaccharides, disaccharides, oligosaccharides
  (2 to 10 units), polysaccharides
• Position of carbyonil (CO) group
• Aldose if terminally located
• Ketose if centrally located
• Reducing property
• Reducing sugars (all monosaccharides)
• Nonreducing sugars (sucrose)

7
STRUCTURAL PROJECTIONS OF MONOSACCHARIDES

• FISCHER by Emil Fischer
• (Nobel Prize in Chemistry 1902)
• 2-D representation for showing
• the configuration of a stereocenter
• Horizontal lines project forward
• while vertical lines project towards
• the rear
• D (R or ) or L (S or -)

8

• HAWORTH by Walter Haworth
• (Nobel Prize in Chemistry 1937)
• A way to view furanose (5-membered ring) and
  pyranose (6-membered ring) forms of
  monosaccharides
• The ring is drawn flat and viewed through its
  edge with the anomeric carbon on the the right
  and the oxygen atom on the rear

9
ANOMERIC CARBON
10
CHAIR BOAT CONFORMATIONS
11
AMINO SUGARS
12
(No Transcript)
13
POLYSACCHARIDES

• STARCH
• Storage carbohydrate in plants
• Two principal parts are amylose (20-25)
  amylopectin (75-80) which are completely
  hydrolyzed to D-glucose
• Amylose is composed of continuous, unbranched
  chain of 4000 D-glucose linked via a 1-4 bonds
• Amylopectin is a chain of 10,000 D-glucose units
  linked via a 1-4 bonds but branching of 24-30
  glucose units is started via a 1-6 bonds

14

• GLYCOGEN
• Energy-reserve carbohydrate in animals
• Highly branched containing approximately 106
  glucose units linked via a 1-4 bonds a 1-6
  bonds
• Well-nourished adult stores 350 g. of it equally
  divided between the liver and muscles

15
CELLULOSE

• Plant skeletal polysaccharide
• Linear chain of 2200 glucose units linked via ß
  1-4 bonds
• High mechanical strength is due to aligning of
  stiff fibers where hydroxyl form hydrogen bonding

16
ACIDIC POLYSACCHARIDES

• Also called mucopolysaccharides (MPS) or
  glycosaminoglycans (GAG)
• Polymers which contain carboxyl groups and/or
  sulfuric ester groups
• Structural and functional importance in
  connective tissues
• Interact with collagen to form loose or tight
  networks

17
ACIDIC POLYSACCHARIDES

• HYALURONIC ACID
• Simplest GAG
• Contains 300-100,000 repeating units of
  D-glucuronic acid and N-acetyl-D-glucosamine
• Abundant in embryonic tissues, synovial fluid,
  and the vitreous humor to hold retina in place
• Joint lubricant shock absorber
• HEPARIN
• Heterogeneous mixture of variably sulfonated
  chains
• Stored in mast cells of the liver, lungs and the
  gut
• Naturally-occurring anticoagulant by acting as
  antithrombin III and antithromboplastin
• Composed of two disaccharide repeating units A
  B
• A is L-iduronic acid-2-sulfate linked to
  2-deoxy-2-sulfamido-D-galactose-6-sulfate
• B is D-glucuronic acid beta-linked to
  2-deoxy-2-sulfamido-D-glucose-6-sulfate

18

• HEPARAN SULFATE
• CHONDROITIN SULFATE
• Most abundant in mammalian tissues
• Found in skeletal and soft connective tissues
• Composed of repeating units of N-acetyl
  galactosamine sulfate linked beta1-4 to
  glucuronic acid
• KERATAN SULFATE
• DERMATAN SULFATE
• Found in skin, blood vessels, heart valves,
  tendons, aorta, spleen and brain
• The disaccharide repeating units are L-iduronic
  acid and N-acetylgalactosamine-4-sulfate with
  small amounts of D-glucuronic acid

19
(No Transcript)
20
GLYCOLYSIS

• The specific pathway by which the body gets
  energy from monosaccharides
• First stage is ACTIVATION
• At the expense of 2ATPs glucose is phosphorylated
• Step 1
• formation of glucose-6-phosphate
• Step 2
• isomerization to fructose-6-phosphate

21

• Step 3
• Second phosphate group is attached to yield
  fructose-1,6-bisphosphate
• Second stage is C6 to 2 molecules of C3
• Step 4
• Fructose-1,6-bisphosphate is broken down into two
  C3 fragments
• glyceraldehyde-3-phosphate (G-3-P) and
• dihydroxyacetone phosphate (DHAP)
• Only G-3-P is oxidized in glycolysis. DHAP is
  converted to G-3-P as the latter diminishes.

22
ATP-YIELDING Third stage

• Step 5
• Glyceraldehyde-3-phosphate is oxidized to
  1,3-bisphosphoglycerate hydrogen of aldehyde is
  removed by NAD
• Step 6
• Phosphate from the carboxyl group is transferred
  to the ADP yielding ATP and 3-phosphoglycerate
• Step 7
• Isomerization of 3-phosphoglycerate to
  2-phosphoglycerate

23

• Step 8
• Dehydration of 2-phosphoglycerate to
  phosphoenolpyruvate (PEP)
• Step 9
• Removal of the remaining phosphate to yield ATP
  and pyruvate
• Step 10
• Reductive decarboxylation of pyruvate to produce
  ethanol and CO2

24
REACTIONS OF GLYCOLYSIS
STEP REACTION ENZYME REACTION TYPE ?G in kJ/mol
1 Glucose ATP ? G-6-P ADP H Hexokinase Phosphoryl transfer -33.5
2 G-6-P ?? F-6-P Phosphoglucose isomerase Isomerization -2.5
3 F-6-P ATP ? F-1,6-BP ADP H Phosphofructo-kinase Phosphoryl transfer -22.2
25
STEP REACTION ENZYME REACTION TYPE ?G in kJ/ mol
4 F-1,6-BP ?? DHAP GAP Aldolase Aldol cleavage -1.3
5 DHAP ?? GAP Triose phosphate isomerase Isomerization 2.5
6 GAP Pi NAD ?? 1,3-BPG NADH H Glyceraldehyde-3-Phosphate Dehydrogenase Phosphorylation coupled to oxidation 2.5
7 1,3-BPG ADP ?? 3-phosphoglycerate ATP Phosphoglycer-ate kinase Phosphoryl transfer 1.3
8 3-phosphoglycerate ?? 2-phosphoglycerate Phosphoglyce-rate mutase Phosphoryl shift 0.8
9 2-phosphoglycerate ?? PEP HOH Enolase Dehydration -3.3
10 PEP ADP H ? pyruvate ATP Pyruvate kinase Phosphoryl transfer -16.7
26
CITRIC ACID CYCLE
STEP REACTION ENZYME PROSTHETIC GROUP REACTION TYPE ?Go in kJ/ mol
1 acetylCoA oxaloacetate HOH ? citrate CoA H Citrate synthase Condensation -31.4
2a Citrate ?? cis-aconitate HOH Aconitase Fe-S Dehydration 8.4
2b Cis-Aconitate HOH ?? isocitrate Aconitase Fe-S Hydration -2.1
27
CITRIC ACID CYCLE
STEP REACTION ENZYME PROSTHETIC GROUP REACTION TYPE ?Go in kJ/ mol
3 Isocitrate NAD ?? a-ketoglutarate CO2 NADH Isocitrate Dehydro-genase Decarboxylation oxidation - 8.4
4 a-ketoglutarate NAD CoA ?? succinyl CoA CO2 NADH a-ketogluta-rate dehydro-genase complex Lipoic acid, FAD, TPP Decarboxyla-tion oxidation -30.1
5 Succinyl CoA Pi GDP ?? succinate GTP CoA Succinyl CoA synthet-ase Substrate-level phosphoryla-tion -3.3
28
CITRIC ACID CYCLE
STEP REACTION ENZYME PROSTHETIC GROUP REACTION TYPE ?Go in kJ/ mol
6 Succinate FAD (enzyme-bound) ?? fumarate FADH2 (enzyme-bound) Succinate dehydro-genase FAD, Fe-S Oxidation 0
7 Fumarate HOH ?? L-malate Fumarase Hydration -3.8
8 L-malate NAD ?? oxaloacetate NADH H Malate dehydro-genase Oxidation 29.7
29
REGULATION OF TCA CYCLE
Pyruvate
- ATP, acetyl CoA NADH
Acetyl CoA
Oxaloacetate
Citrate
Malate
Isocitrate
- ATP NADH ADP
Fumarate
ICD
?-Ketoglutarate
Succinate
- ATP, succinyl CoA NADH
?-KGD
Succinyl CoA
30
BIOSYNTHETIC ROLES OF TCA CYCLE
Pyruvate
Other amino acids, purines pyrimidines
Acetyl CoA
Oxaloacetate
Citrate
Fatty acids, sterols
Malate
Aspartate
Isocitrate

Fumarate
Other amino acids purines
?-Ketoglutarate
Succinate
Porphyrins, heme, chlorophyll
Glutamate
Succinyl CoA
31
NOTES TO REMEMBER

• The unusual thing about the structure of
  N-acetylmuramic acid compared to other
  carbohydrates is the presence of a lactic acid
  side chain.
• Cell walls of plants are cellulosic (polymer of
  D-glucose) bacterial cell walls consist mainly
  of polysaccharide crosslinked to peptide through
  murein bridges and fungal cell walls are
  chitinous (polymer of N-acetyl-ß-D-glucosamine)

32

• Glycogen and starch differ mainly in the degree
  of chain branching.
• Enantiomers are nonsuperimposable, mirror-image
  stereoisomers differing configuration on all
  carbons while diastereomers are nonsuperimposable
  nonmirror-image stereoisomers differing only on
  two carbons.
• Fischer projection of glucose has 4 chiral
  centers while its Haworth projection has 5 chiral
  centers.

33

• Sugar phosphate is an ester bond formed between a
  sugar hydroxyl and phosphoric acid.
• A glycosidic bond is an acetal which can be
  hydrolyzed to regenerate the two original sugar
  hydroxyls.
• A reducing sugar is one that has a free aldehyde
  group that can be easily oxidized.

34

• Major biochemical roles of glycoproteins are
  signal transduction as hormones, recognition
  sites for external molecules in eukaryotic cell
  membranes, and defense as immunoglobulins.
• L-sorbitol is made by reducing D-glucose.
• Arabinose is a ribose epimer, thus, its
  derivatives ara-A and ara-C if substituted for
  ribose act as inhibitors in reactions of
  ribonucleosides.

35

• Two best precursors for glycogen are glucose and
  fructose.
• Cellulose because of the ß- bonding is linear as
  to structure and structural as to role while
  starch because of a-bonding coils with energy
  storage role.
• The highly branched nature of glycogen gives rise
  to a number of available glucose molecules at a
  time upon hydrolysis to provide energy. A linear
  one provides one glucose at a time.

36

• The enzyme ß-amylase is an exoglycosidase
  degrading polysaccharides from the ends. The
  enzyme a-amylase is an endoglycosidase cleaving
  internal glycosidic bonds.
• Dietary fibers bind toxic substances in the gut
  and decreases the transit time, so harmful
  compounds such as carcinogens are removed from
  the body more quickly than would be the case with
  low-fiber diet.
• The sugar portions of the blood group
  glycoproteins are the source of the antigenic
  difference.

37

• Cross-linking can be expected to play a role in
  the structures of cellulose and chitin where
  mechanical strength is afforded by extensive
  hydrogen bonding.
• Converting a sugar to an epimer requires
  inversion of configuration at a chiral center.
  This can only be done by breaking and reforming
  covalent bonds.
• Vitamin C is a lactone (a cyclic ester) with a
  double bond between two of the ring carbons. The
  presence of a double bond makes it susceptible to
  air oxidation.
• The sequence of monomers in a polysaccharide is
  not genetically coded and in this sense does not
  contain any information unlike the nucleotide
  sequence.

38

• Glycosidic bonds can be formed between the side
  chain hydroxyls of serine or threonine residues
  and the sugar hydroxyls. In addition, there is a
  possibility of ester bonds forming between the
  side chain carboxyl groups of aspartate or
  glutamate and the sugar hydroxyls.
• In glycolysis, reactions that require ATP are
• 1. phosphorylation of glucose (HK,GK)
• 2. phosphorylation of fructose-6-phosphate
  (PFK)
• Reactions that produce ATP are
• 1. transfer of phosphate from 1,3-
• bisphosphoglycerate to ADP (PGK)
• 2. transfer of phosphate from PEP to ADP (PK)

39

• In glycolysis, reactions that require NADH are
• 1. reduction of pyruvate to lactate (LDH)
• 2. reduction of acetaldehyde to ethanol
• (alcohol dehydrogenase)
• Reactions that require NAD are
• 1. oxidation of G-3-P to give 1,3-DPG (G-3-PD)
• NADH-linked dehydrogenases are LDH, ADH G-3-PD.
• The purpose of the step that produces lactate is
  to reduce pyruvate so that NADH can be oxidized
  to NAD needed for the step catalyzed by
  glyceraldehyde-3-phosphate.

40

• Aldolase catalyzes the reverse aldol condensation
  of fructose-1,6-bisphosphate to
  glyceraldehyde-3-phosphate and DHAP.
• The energy released by all the reactions of
  glycolysis is 184.5 kJ mol glucose/mol. The
  energy released by glycolysis drives the
  phosphorylation of two ADP to ATP for each
  molecule of glucose, trapping 61.0 kJ
  mol/glucose. The estimate of 33 efficiency comes
  from the calculation (61.0/184.5) x 100 33.
• There is a net gain of two ATP molecules per
  glucose molecule consumed in glycolysis. The
  gross yield of 4 ATPs per glucose molecule, but
  the reactions of glycolysis require two ATP per
  glucose.

41

• Pyruvate can be converted to lactate, ethanol or
  acetylCoA.
• The free energy of hydrolysis of a substrate is
  the energetic driving force in substrate-level
  phosphorylation. An example is the conversion of
  glyceraldehyde-3-phosphate to 1,3-bisphosphoglycer
  ate.
• Coupled reactions in glycolysis are those
  reactions catalyzed by hexokinase,
  phosphofructokinase, glyceraldehyde-3-phosphate
  dehydrogenase, phosphoglycerokinase, and pyruvate
  kinase.

42

• Isozymes allow for subtle control of the enzyme
  to respond to different cellular needs. For
  example, in the liver, LDH is most often used to
  convert lactate to pyruvate, but the reaction is
  often reversed in the muscles. Having a different
  isozyme in the liver and the muscle allows for
  those reactions to be optimized.
• Fructose-1,6-bisphosphate can only undergo the
  reactions of glycolysis. The components of the
  pathway up to this point can have other metabolic
  fates.
• The physiologically irreversible glycolytic steps
  are those catalyzed by HK, PFK and PK. Thus, they
  are controlling points in glycolysis.

43

• Hexokinase is inhibited by glucose-6-phosphate.
  Phosphofructokinase is inhibited by ATP and
  citrate. Pyruvate kinase is inhibited is
  inhibited by ATP, acetylCoA and alanine.
• Phosphofructokinase is stimulated by AMP and
  fructose-2,6-bisphosphate.
• Pyruvate kinase is stimulated by AMP and
  fructose- 1,6-bisphosphate.
• An isomerase is a general term for an enzyme that
  changes the form of a substrate without changing
  its empirical formula.
• A mutase is an enzyme that moves a functional
  group such as a phosphate to a new location in a
  substrate molecule.

44

• The glucokinase has a higher Km for glucose than
  hexokinase. Thus, under conditions of low
  glucose, the liver will not convert glucose to
  glucose-6-phosphate, using a substrate that is
  needed elsewhere. When the glucose concentration
  becomes higher, however, glucokinase will
  function to help phosphorylate glucose so that it
  can be stored as glycogen.
• The net yield of ATP from glycolysis is the same,
  2 ATP, when either fructose, mannose, and
  galactose is used. The energetics of the
  conversion of hexoses to pyruvate are the same
  regardless of hexose type.
• The net yield of ATP is 3 from glucose derived
  from glycogen because the starting material is
  glucose-1-phosphate. One of the priming reactions
  is no longer used.

45

• A reaction with a negative ?Go is
  thermodynamically possible under standard
  conditions.
• Individuals who lack the gene that directs the
  synthesis of the M form of the enzyme PFK can
  carry on glycolysis in their livers but suffer
  muscle weakness because they lack the enzyme in
  muscle.
• The reaction of 2-PG to PEP is a dehydration
  (loss of water) rather than a redox reaction.
• The hexokinase molecule changes shape drastically
  on binding to substrate, consistent with the
  induced fit theory of an enzyme adapting itself
  to its substrate.

46

• ATP is an inhibitor of several steps of
  glycolysis as well as other catabolic pathways.
  The purpose of catabolic pathways is to produce
  energy, and high levels of ATP mean the cell
  already has sufficient energy. G-6-P inhibits HK
  and is an example of product inhibition. If G-6-P
  level is high, it may indicate that sufficient
  glucose is available from glycogen breakdown or
  that the subsequent enzymatic steps of glycolysis
  are going slowly. Either way there is no reason
  to produce more G-6-P.
• Phosphofructokinase is inhibited by a special
  effector molecule, fructose-2,6-bisphosphate,
  whose levels are controlled by hormones. It is
  also inhibited by citrate, which indicates that
  there is sufficient energy from the TCA cycle
  probably from fat or amino acid catabolism.

47

• PK is also inhibited by acetylCoA, the presence
  of which indicates that fatty acids are being
  used to generate energy for the citric acid
  cycle.
• The main function of glycolysis is to feed carbon
  units to the TCA cycle. When these carbon
  skeletons can come from other sources, glycolysis
  is inhibited to spare glucose for other purposes.
• Thiamine pyrophosphate (TPP) is a coenzyme in the
  transfer of 2-carbon units. It is required for
  catalysis by pyruvate decarboxylase in alcoholic
  fermentation. The important part of TPP is the
  five-membered ring where a C is found between an
  S and N. This carbon forms a carbanion and is
  extremely reactive, making it able to perform
  nucleophilic attack on carbonyl groups leading to
  decarboxylation of several compounds in different
  pathways.

48

• TPP is a coenzyme required in the reaction
  catalyzed by pyruvate carboxylase. Because this
  reaction is a part of the metabolism of ethanol,
  less will be available to serve as a coenzyme in
  the reactions of other enzymes that require it.
• Animals that have been run to death have
  accumulated large amounts of lactic acid in their
  muscle tissue, accounting for the sour taste of
  their meat.
• Conversion of glucose to lactate rather than
  pyruvate recycles NADH.
• The formation of fructose-1,6-bisphosphate is the
  committed step in glycolysis. It is also one of
  the energy-requiring steps of the said pathway.

49

• A positive ?Go does not necessarily mean that the
  reaction has a positive ?G. Substrate
  concentrations can make a negative ?G out of a
  positive ?Go.
• The entire pathway can be looked at as a large
  coupled reaction. Thus, if the overall pathway
  has a negative ?G, an individual step may be able
  to have a positive ?G and the pathway can still
  continue.

50

• In glycogen storage, the reactions that require
  ATP are
• 1. formation of UDP-glucose from
  glucose-1-phosphate and UTP (indirect
  requirement since ATP is needed to regenerate
  UTP) (UDP-glucose phosphorylase)
• 2. regeneration of UTP (nucleoside phosphate
  kinase)
• 3. carboxylation of pyruvate to oxaloacetate
  (pyruvate carboxylase)
• Reactions that produce ATP are NONE.
• Three differences between NADPH and NADH
• 1. phosphate at 2 position of ribose in NADPH
• 2. NADH is produced in oxidative reactions that
  yield ATP while NADPH is a reducing agent in
  biosynthesis.
• 3. Different enzymes use NADH as a coenzyme
  compared to those that require NADPH.

51

• In glycogen storage, there is no reaction that
  requires acetylCoA but biotin is required in the
  carboxylation of pyruvate to oxaloacetate.
• The four fates of glucose-6-phosphate are
• Converted to glucose (gluconeogenesis)
• Converted to glycogen (glycogenesis)
• Converted to pentose phosphates
• Hydrolyzed to pyruvate (glycolysis)

52

• In making equal amounts of NADPH and pentose
  phosphates, it only involves oxidative reactions.
  In making mostly or purely NADPH, the use of
  oxidative reactions, transketolase and
  transaldolase reactions, and gluconeogenesis are
  required. In making mostly or only pentose
  phosphates, needed reactions are transketolase,
  transaldolase, and glycolysis in reverse.
• Transketolase catalyzes the transfer of 2-carbon
  unit, whereas transaldolase catalyzes the
  transfer of a 3-carbon unit.
• It is essential that the mechanisms that activate
  glycogen synthesis also deactivate glycogen
  phosphorylase because they both occur in the same
  cell compartment. If both are on at the same
  time, a futile ATP hydrolysis results. On/off
  mechanism is highly efficient in its control.

53

• UDPG, in glycogen biosynthesis, transfers glucose
  to the growing glycogen molecule.
• Glycogen synthase is subject to covalent
  modification and to allosteric control. The
  enzyme is active in its phosphorylated form and
  inactive when dephosphorylated.
• AMP is an allosteric inhibitor of glycogen
  synthase, whereas ATP and glucose-6-phosphate are
  allosteric activators.
• In gluconeogenesis, biotin is the molecule to
  which carbon dioxide is attached to the process
  of being transferred to pyruvate. The reaction
  produces oxaloacetate, which then undergoes
  further reactions of gluconeogenesis. Biotin is
  not used in glycogenesis and PPP.

54

• In gluconeogenesis, glucose-6-phosphate is
  dephosphorylated to glucose (last step) in
  glycolysis, G-6-P isomerizes to
  fructose-6-phosphate (early step).
• The Cori cycle is a pathway in which there is
  cycling of glucose due to glycolysis in muscle
  and gluconeogenesis in liver. The blood
  transports lactate from muscle to liver and
  glucose from liver to muscle.
• There is a net gain of 3, rather than 2, ATP when
  glycogen, not glucose, is the starting material
  of glycolysis.

55

• Control mechanisms are important in metabolism.
  They are
• Allosteric control (takes place in msec)
• Covalent control (takes place from s to min)
• Genetic control ( longer time scale)
• Enzymes, like all catalysts, speed up the forward
  and reverse reaction to the same extent. Having
  different catalysts is the only way to ensure
  independent control over the rates of the forward
  and the reverse process.
• The glycogen synthase is an exergonic reaction
  overall because it is coupled to phosphate ester
  hydrolysis.

56

• Increasing the level of ATP is favorable to both
  gluconeogenesis and glycogen synthesis.
• Decreasing the level of fructose-1,6-bisphosphate
  would tend to stimulate glycolysis, rather than
  gluconeogenesis and glycogen synthesis.
• If a cell needs NADPH, all the reactions of the
  PPP take place. If a cell needs
  ribose-5-phosphate, the oxidative portion of the
  pathway can be bypassed and only the nonoxidative
  reshuffling reactions take place. The PPP does
  not have a significant effect on the ATP supply
  of a cell.
• Glucose-6-phosphate is expectedly oxidized to a
  lactone rather than an open-chain ester because
  the latter is easy to hydrolyze.

57

• In the PPP resshuffling reactions, without an
  isomerase, all the sugars involved are keto
  sugars that are not substrates for transaldolase.
  
• Sugar nucleotides (UDPG) have two phosphates
  which when hydrolyzed drives towards the
  polymerization of glycogen. Thus, they are fit
  for glycogenesis.